In the manufacture of concrete, the concrete is usually cast using a concrete form which takes the shape of the form. The wet concrete is poured into or against the concrete form and, upon setting and removal of the form, the newly-exposed concrete surface is a reverse impression of the inner surface of the form. In the case of wooden forms, the concrete takes the appearance of the wood grain; and in the case of forms involving seamed form members, the concrete shows any seams which have not been sufficiently masked.
Air is often added to a concrete mix and water is often added in excess of the amount required for hydration. Such air and water are useful to render the mix flowable and to facilitate handling and pouring. However, the excess water, if left undrained, results in concrete having a weakened surface and, the air, if not removed, results in surface pores as large as 0.1 to 3 cm, which pores leave an uneven surface open to the effects of dirt and erosion by the freeze-thaw cycles of water.
Examples of prior art concrete forms include:
U.S Pat. No. 4,730,805 (Yokota et al.) which discloses a form for forming concrete which utilizes a support and at least two layers of fabric over the support. The support can have lugs to space the fabric from the support and the fabric layers and the lugs assist in draining water away from the curing concrete. The support may have drainage holes for removal of excess water and air. The fabric is bonded to the support and is stiff and immovable relative to the support.
U.S. Pat. No. 4,856,754 (Yokota et al.) which discloses a concrete form using double-woven fabrics on a support plate with holes to provide water drainage. One woven fabric is adhered to the plate and the other woven fabric is sewn to the first.
U.S. patent application Ser. No. 07/472,902 filed Jan. 31, 1990, now abandoned which discloses a form for patterned concrete comprising a support means, a grid having interconnected spacing members which form holes in the grid having an individual area of at least 0.25 cm.sup.2 and at least a portion of which rests against the support means, and a porous fabric juxtaposed with the grid and set apart from the support by the grid. The fabric generally has a pore size of between 10 to 250 microns on each side so that a number of fine concrete particles (typically 30 to 90 microns) can enter and fill the fabric's open spaces and so that excess water and air can pass therethrough.
Fine concrete particles typically fill the fabric's larger pores, especially if excessive concrete compaction occurs. Usually, if enough fine concrete particles have entered the fabric structure and sufficient concrete curing is allowed, then the separation of the fabric from the cured concrete becomes very difficult or even impossible. This occurs because the concrete particles that have entered the fabric and hardened therein pull the fabric fibers out of the surface of the fabric when the fabric is separated from the concrete. The problem becomes worse when the fabric is reused with loose surface fibers since the loose fibers tend to become embedded in the cured concrete thereby causing delamination of the fabric web. The problem is heightened if the fabric is not handled with care during form assembly and disassembly, since mechanical friction (e.g., rubbing) tends to make the fabric fuzzy and causes the loose fibers to stick to the concrete. Multiple use of the fabric forms causes more of the fabric pores to become plugged by fine concrete particles resulting in greatly reduced levels of water and air evacuation.
Clearly, what is needed is an improved concrete form and concrete form liner which are both sufficiently reusable and which do not have the deficiencies inherent in the prior art. Specifically, the improved form should have a fabric liner which does not easily stick to the concrete surface and which prevents substantially all concrete particles from passing through the liner, yet which is sufficiently open to permit the passage of excess water and air. Other objects and advantages of the present invention will become apparent to those skilled in the art upon reference to the attached drawings and to the detailed description of the invention which hereinafter follows.